System for determining relative air humidity

ABSTRACT

An arrangement for determining relative atmospheric humidity and temperature by means of a capacitive atmospheric humidity sensor and associated sensor evaluation electronics. The capacitive atmospheric humidity sensor has a polymer layer as a dielectric and two electrically conductive electrodes. For the temporary heating up of the capacitive atmospheric humidity sensor, one of the electrically conductive electrodes is designed as a laminar heating resistor with at least two power terminals.

This application is made pursuant to 35 U.S.C. §371 of internationalapplication number PCT/EP 98/03752, filed Jun. 19, 1998, with a prioritydate of Jul. 11, 1997.

FIELD OF THE INVENTION

The invention concerns an arrangement for determining relativeatmospheric humidity by means of a capacitive atmospheric humiditysensor and associated sensor evaluation electronics, the capacitiveatmospheric humidity sensor having a polymer layer as a dielectric andtwo electrically conductive electrodes.

BACKGROUND OF THE INVENTION

Such an arrangement is known, for example, from DE 28 51 686, thedielectric properties of this polymer layer representing a function ofthe relative atmospheric humidity. The determination of the relativeatmospheric humidity in this case takes place by measuring thecapacitance between the two electrically conductive electrodes. Animprovement in the speed and accuracy of the measurement, in particularwhen there is high relative humidity, is to be achieved in this case byheating up the polymer layer.

In the case of such capacitive atmospheric humidity sensors, it has beenfound that the polymer layers become contaminated by foreign substancesin the course of the service life. In particular when there is frequentcondensation (that means an atmospheric humidity of approximately 100%),and when it is cold, residues remain in the layer, so that thefunctional interrelationship between capacitance and atmospherichumidity is disturbed. Long-term instabilities or signal drift are thusthe consequence. If these sensor elements are then heated up, forexample dried out in an oven, it is also possible for the measuredvalues to be stabilized again, with the foreign substances also beingeliminated.

If these atmospheric humidity sensors are used for air-conditioningsystems in buildings, they are usually serviced or replaced regularly.If, however, the atmospheric humidity sensors are used inair-conditioning systems of motor vehicles, they must operate underaggravated conditions without servicing, if possible over a period ofyears.

Thus there is a need for an arrangement for determining relativeatmospheric humidity of the type stated at the beginning which operatessatisfactorily without any additional servicing effort.

Other needs will become apparent upon a further reading of the followingdetailed description taken in conjunction with the drawings.

SUMMARY OF THE INVENTION

The aforementioned needs are fulfilled according to the invention by oneof the electrically conductive, metallized electrodes being designed asa laminar heating resistor with at least two power terminals.

The arrangement according to the invention has the advantage that thecapacitive atmospheric humidity sensor can be heated up outside themeasuring periods in a simple way and at any time, so that anycontaminations there may be can be removed by vaporization.

In the case of the arrangement according to the invention, it may beprovided that the electrically conductive electrodes are arranged onboth sides of the polymer layer, opposite one another, or on one side ofthe polymer layer, alongside one another.

It is particularly advantageous if the electrodes of the capacitiveatmospheric humidity sensor consist of platinum, which has a relativelyhigh resistivity and, in the form of a thin foil, is consequentlyparticularly suited as a heating resistor. In principle, some othernoble metal can also be used.

In the case of the preferred exemplary embodiment, it is thereforepossible to make the foil layer thickness of the platinum electrodes sosmall that moisture can penetrate through.

For the heating up of the capacitive atmospheric humidity sensor, theplatinum electrode designed as a heating resistor is supplied withcurrent in specific time cycles, it having been found to be advantageousif this takes place each time that the arrangement is switched on.

In the case of another embodiment of the invention, it is proposed thatthe heating resistor is supplied with current for a short time only whenthe ambient temperature is below a fixed predetermined value when thedevice is switched on. Provided here for sensing the ambient temperatureis a temperature sensor which is connected to the sensor evaluationelectronics and emits an appropriate compensating variable fordetermining the exact relative atmospheric humidity by the sensorevaluation electronics.

A development of the invention is that the heating resistor is in eachcase supplied with current only after a fixed predetermined waitingtime, which has proven to be particularly cost-effective andcost-saving.

Use of the arrangement according to the invention in an air-conditioningsystem, preferably of motor vehicles, is particularly advantageous,because such air-conditioning systems operate without servicing over aperiod of years.

These and other features and advantages of the invention will beapparent upon consideration of the following detailed description of thepreferred embodiment of the invention, taken in conjunction with theappended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side view of an atmospheric humidity sensor,

FIG. 2 shows a plan view of an atmospheric humidity sensor,

FIG. 3 shows an arrangement for the operation of an atmospheric humiditysensor and

FIGS. 4a) to c) show flow diagrams of various heating cycles for theatmospheric humidity sensor.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention is capable of embodiment in various forms,there is shown in the drawings and will be hereinafter described apresently preferred embodiment with the understanding that the presentdisclosure is to be considered as an exemplification of the invention,and is not intended to limit the invention to the specific embodimentdescribed and illustrated.

The capacitive atmospheric humidity sensor 1 represented in FIGS. 1 and2 essentially comprises a polymer layer 2 as a dielectric and twomutually opposite electrically conductive (metallized) electrodes 3 and4. According to another embodiment, these electrodes 3 and 4 may alsolie on the dielectric 2 alongside one another. The electrodes 3 and 4are provided with power terminals 6 and 7, 8, to which supply leads 9and 11, 12 are connected. Since the electrode 4 is designed according tothe invention as a heating resistor, two power terminals 7 and 8 areprovided for temporarily supplying heating current, while the electrode3 has only one power terminal 6. On the one hand, a heating current canbe supplied via the two power terminals 7 and 8—outside the measuringperiods—and on the other hand—during the measuring times—a measuringsignal can at the same time be picked off via the power terminal 6.

As already stated above, the capacitive atmospheric humidity sensor 1serves for determining relative atmospheric humidity, the supply leads 9of the electrode 3 and 11, 12 of the electrode 4 according to FIG. 3being connected to sensor evaluation electronics 13. Also connected tothese sensor evaluation electronics 13 is a temperature sensor 14 fordetermining the ambient temperature via leads 16, 17. Consequently,atmospheric humidity measurement can take place in the sensor evaluationelectronics 13 by measuring the capacitance of the capacitiveatmospheric humidity sensor 1 and measuring the temperature byevaluating the signal supplied by the temperature sensor 14, whereby acompensating variable is generated at the same time for thedetermination of the exact relative atmospheric humidity.

Then the measuring signal for temperature and atmospheric humidity canbe picked off at the output 18 of the sensor evaluation electronics 13and can be displayed by means of a measuring instrument, not representedin FIG. 3. Both a current for operating the sensor evaluationelectronics 13 and a heating current for the capacitive atmospherichumidity sensor 1 can be supplied via the supply leads 21, 22.

In FIG. 4, various operating modes of the arrangement according to theinvention for the heating up of the capacitive atmospheric humiditysensor 1 are represented on the basis of flow diagrams according to a),b) and c). These flow diagrams may be stored, for example, as a programin a microprocessor 23 of the sensor evaluation electronics.

In the case of the flow diagram according to FIG. 4a), after theswitching on (A) of the arrangement according to the invention byclosing the switch 24 (in FIG. 3), the heating of the capacitiveatmospheric humidity sensor 1 is at the same time switched on (B). Whenchecking the heating time (C) , either a “yes” or “no” is emitted asinformation, according to the duration, and the heating is interrupted(D) or continued. After the switching off of the heating, themeasurement of temperature and humidity (E) then takes place.

The flow diagram according to FIG. 4b) differs from the flow diagramaccording to FIG. 4a) only in that, before the switching on of theheating (B), a measurement of the ambient temperature T and a comparisonwith a predetermined temperature value T0 (of for example +10° C.) takesplace (F). Only if the ambient temperature T is below the predeterminedtemperature value T0 is the heating operation then initiated.

In the case of the flow diagram according to FIG. 4c), a component (G)“waiting time elapsed?” is inserted in addition to the conditions of theflow diagrams according to FIG. 4a) and FIG. 4b) . In this case, theheating is only switched on when a certain waiting time (for example 10hours) has elapsed. This may also be linked with a predeterminedtemperature value T0.

The foregoing description of a preferred embodiment of the invention hasbeen presented for purposes of illustration and description, and is notintended to be exhaustive or to limit the invention to the precise formdisclosed. The description was selected to best explain the principlesof the invention and their practical application to enable othersskilled in the art to best utilize the invention in various embodimentsand various modifications as are suited to the particular usecontemplated. It is intended that the scope of the invention not belimited by the specification, but be defined by the claims set forthbelow.

What is claimed is:
 1. An arrangement for determining relativeatmospheric humidity comprising a capacitive atmospheric humidity sensorand associated sensor evaluation electronics the capacitive atmospherichumidity sensor having a polymer layer as a dielectric and twoelectrically conductive electrodes wherein one of the electricallyconductive, metallized electrodes is designed as a laminar heatingresistor with at least two power terminals.
 2. The arrangement asclaimed in claim 1, wherein the electrically conductive electrodes arearranged on both sides of the polymer layer opposite one another.
 3. Thearrangement according to claim 1, wherein the electrically conductiveelectrodes are arranged on one side of the polymer layer alongside oneanother.
 4. The arrangement as claimed in claim 1, wherein theelectrically conductive electrodes consist of platinum.
 5. Thearrangement as claimed in claim 1, wherein the layer thickness of theelectrodes is so small that they are liquid-permeable.
 6. Thearrangement as claimed in claim 1, wherein the heating resistor issupplied with current, and is consequently heated up, in specific timecycles.
 7. The arrangement as claimed in claim 6, wherein the heatingresistor is supplied with current for a short time each time thearrangement is switched on.
 8. The arrangement as claimed in claim 6,wherein the heating resistor is supplied with current for a short timeonly when the ambient temperature is below a fixed predetermined valuewhen the device is switched on.
 9. The arrangement as claimed in claim7, wherein the heating resistor is supplied with current in each caseonly after a fixed predetermined waiting time.
 10. The arrangement asclaimed in claim 8, wherein, for sensing the ambient temperature, thereis provided a temperature sensor which is connected to the sensorevaluation electronics and emits an appropriate compensating variablefor determining the exact relative atmospheric humidity by the sensorevaluation electronics.
 11. The arrangement as claimed in one or more ofclaim 1, which comprises integration thereof in an air-conditioningsystem, in particular for motor vehicles.